In a conventional power module used to control a large current, for example, in an electric vehicle, an electric train, a machine tool or the like, a metal-ceramic insulating substrate is fixed on one surface of a metal plate or a composite material called a base plate by soldering, and an electronic component such as a semiconductor chip is fixed on the metal-ceramic insulating substrate by soldering. Further, a heat radiating fin or a cooling jacket made of metal is attached to another surface (a rear surface) of the base plate via a heat conductive grease by screwing or the like.
Since the soldering of the base plate, the electronic component and so on to the metal-ceramic insulating substrate is performed by heating, warpage of the base plate is likely to occur due to the difference in thermal expansion coefficient between bonded members in soldering. Further, the heat generated from the electronic component and so on is radiated to air, a cooling water or the like by the heat radiating fin or the cooling jacket via the metal-ceramic insulating substrate, the solder, and the base plate, so that if the aforementioned warpage of the base plate occurs, a clearance when the heat radiating fin or the cooling jacket is attached to the base plate increases to extremely decrease the heat radiation performance.
Hence, for example, Patent Document 1 discloses a metal-ceramic bonded substrate in which heat radiating fins (a reinforcing part) and a metal base plate are integrally formed and the warpage of the base plate that is the above-described problem can be made extremely small. Further, for example, Patent Document 2 and Patent Document 3 disclose a cooling jacket attached to the metal base plate, the heat radiating fins or the like to efficiently cool the heat generator.
Accordingly, by attaching the cooling jacket described in Patent Document 2, 3 to the metal-ceramic bonded substrate described in Patent Document 1, the warpage of the metal base plate is suppressed, resulting in a metal-ceramic bonded substrate with extremely high cooling efficiency.
On the other hand, Patent Document 4 discloses a grooving processing method of performing grooving processing on a work material block located on a base member (a metal base plate) in a heat sink or the like for cooling a semiconductor device and so on. The grooving processing method enables performance of grooving without occurrence of burr at the work piece block.    Patent Document 1: Japanese Patent Application Laid-open No. 2008-218938    Patent Document 2: Japanese Patent Application Laid-open No. 2006-324647    Patent Document 3: Japanese Patent Application Laid-open No. 2008-135757    Patent Document 4: Japanese Patent Application Laid-open No. 2009-56520